Field of the Invention
The present invention relates to an X-ray imaging system and an image processing method.
Description of Related Art
Radiographic imaging systems for X-ray phase imaging are known that include gratings disposed between radiation sources and radiation detectors. Techniques are also known that detect defects in pixels caused by deformed gratings in such radiographic imaging systems (for example, refer to Patent document 1 (Japanese Patent No. 5378335)).
Conventional absorption images include regions unsuitable for imaging, such as regions outside the radiation field (low-intensity X-ray regions), where an amount of X-rays to reach the radiation detector is insufficient and thereby object signals are absent (see FIG. 21C). Such low-intensity X-ray regions are detected through radiation-field recognition of the absorption images and are usually masked.
Differential phase images and small-angle scattering images acquired through phase-contrast imaging with gratings are unsatisfactory for imaging because object signals are absent due to a lack of a moire stripe pattern not only in the low-intensity X-ray regions but also in the out-of-grating regions through which incident X-rays do not pass through grating structure portions of all gratings. In such region, photon errors and detector errors are amplified through computation, causing a large variation in the signal values that generates “salt and pepper noise” (see FIGS. 21A and 21B). Because of containing out-of-grating regions, such regions of salt and pepper noise cannot be correctly recognized through conventional methods based only on X-ray intensity, such as radiation-field recognition. Salt and pepper noise significantly impairs the image quality. Moreover, the time required for diagnosis and examination by medical doctors increases because the doctors must visually distinguish between regions of salt and pepper noise and regions corresponding to the object, and this increases the work burden applied to the operator. The salt and pepper noise contained in reconstructed images (differential phase images, small-angle scattering images, and absorption images) significantly impairs the stability of image processing (post-processing) performed on the images, such as correction and combination, and computation for unnecessary regions increases the processing time.
The radiographic imaging system according to Patent document 1 detects manufacturing defects in pixel units in the in-grating regions and defects in the detector and corrects these defects with peripheral pixels. The regions outside the X-ray irradiation field and the out-of-grating regions remain uncorrected.